Crystal structures and crystal chemistry of AgXPO4 (X = Be, Zn)

The crystal structures of the tetrahedral framework compounds AgBePO4, low-T alpha-AgZnPO4, and high-T beta-AgZnPO4 have been determined by X-ray diffraction using single crystals grown from a silver molybdate flux, AgBePO4 (a = 8.213(3) Angstrom, b = 7.884(3) Angstrom, 14.424(7) Angstrom, beta = 90.20(3)degrees, P2(1)/n, Z = 12, wR(F-2) = 0.091) and beta-AgZnPO4 (a = 8.695(2) Angstrom, b = 8.107(1) Angstrom, c = 15.375(2) Angstrom, beta = 90.38(1)degrees, P2(1)/n, Z = 12, wR(F-2) 0.074) are isostructural and adopt a beryllonite-type framework with a mixed UDUDUD + UUDDUD topology, alpha-AgZnPO4 (a = 10.218(2) Angstrom, c = 7.948(1) Angstrom, P6(3), Z = 8, wR(F-2) = 0.054) crystallizes with a structure similar to those of high-T NaCoPO4 and KZnPO4, with a mixed UDUDUD + UUUDDD framework topology, The phase transformation occurring at 670 degrees C in AgZnPO4 and the absence thereof in AgBePO4 are discussed in terms of the bonding environments around the Ag+ cations in their crystal structures, (C) 1998 Academic Press.